In many power electronics applications, transformers are used on one side or both sides of the converter where the voltage amplitude of the grid or the load is different than the converter input and/or output voltages. Intentional or unintentional time lags between the semiconductors in the converters display a certain imprecision in their switching time. Due to this, asymmetric or uneven operation may occur in the converter. This can cause an undesired direct current flowing through the transformer windings. Since either the system feedback cannot sense this direct current without any measurement device or the system is not current controlled, the DC component results in increased magnetization current, saturate the core and increase the heat and therefore the losses of the transformer.
Although, compared with the amplitude of the AC output voltage this DC offset is very small (for example a few volts or fractions thereof), the DC offset can bring the transformer to saturation. This substantially reduces the power to be transmitted, or can even render operation impossible.
Attempts have been made to measure and correct the DC offset by means of high-precision DC current transformers, which are expensive. However, this likewise produced unsatisfactory correction of the offset, and the control was correspondingly complicated and expensive.
U.S. Pat. No. 5,870,297 discloses the use of a low pass filter followed by a DC measuring unit to generate a measuring signal to correct for the DC offset, which may be used for low to medium voltage applications of less than 3.6 kV.